1. Field of the Invention
The present invention relates to a sound output system and, more particularly, to a sound output system to reproduce a stereophonic sound field wit the high fidelity.
2. Description of the Related Art
In the audio field, an era of CD (compact disc player) and DAT (digital audio tape recorder) has come and what is called a sound quality is remarkably improved. However, when considering the viewpoint of high-fidelity (Hi-Fi) stereos, it is the main stream as usual that with respect to the output sections of the audio signals of the high sound quality, the ideal listening point is located at only the apex of an isosceles triangle which is formed by setting the straight line connecting two speakers to a bottom side. Therefore, the ideal audio system in which the listener can enjoy listening the true Hi-Fi stereophonic sounds in a large listening area is not realized yet. This is because it is mainly considered that the directional distribution of the acoustic energies from the sound output section is not controlled.
In U.K. Patent Application No. 2188811A inventor of this patent application has already proposed a sound output system which can control a directional distribution in a wide area. The present invention relates to the improvement of this prior art. Hitherto, various kinds of techniques have been tried to enlarge the listening area in which the stereophonic feeling can be obtained (hereinafter, referred to as a sweet area). The following prior arts have been known as such conventional techniques.
(1) Diffusion sound field type system of Bose Co., Ltd. in U.S.A.
(2) VSS-70 system for an AV (audio/visual) system of Pioneer Electronic Corporation.
Since the system of the item (1) has been described in detail in the specification of U.K. Patent Application No. 2188811A only the essential points will be shown here.
(1) Drawbacks of the diffusion sound field type speaker system of Bose Co., Ltd. in U.S.A.:
(a) Drawback regarding the phase:
The speaker of Bose Co., Ltd. uses a method whereby the acoustic energies are radiated to both of the front area (for the direct sounds) and the rear area (for the indirect sounds) and the acoustic energies in both of the front and rear areas are used, thereby enlarging the sweet area. Therefore, the phase of the direct sound and the phase of the primary reflection sound from the wall surface on the rear side of the speaker mixedly exist.
(b) Drawback regarding the control of the indirect sound:
In spite of the fact that the primary reflection sound has the main part, the primary reflection sound can be adjusted by only the limited adjusting means such as setting of the speaker or the like. This system cannot cope with a variety of listening rooms.
In consideration of the above drawbacks, the Bose's speaker system is not regarded as a true Hi-Fi stereo of the wide area type.
(2) VSS-70 system of Pioneer Electronic Corporation:
Pioneer Electronic Corporation has developed the VSS-70 system to enlarge the sweet area for the AV. The VSS-70 system has been designed by considering a known sound effect called a Haas effect in a manner such that the central sound source can be localized at the center even at the locations other than the apex of the isosceles triangle (hot spot). Practically speaking, the sound which arrives from the distant speaker with a delay time is enhanced and the influence by the sound which has already arrived early from the near speaker is set off, thereby localizing the sound image at the center. For this purpose, it is inevitable to accurately control the sound pressure at frequencies of a wide frequency band at each listening point. In the case of the stereophonic sound reproduction, the localization of the sound image depends on the sound pressures of the direct sounds which are generated from the right and left speakers. Therefore, it is important to control the directivity at each frequency.
However, in the case of the VSS-70 system, the directivities of the mediant and high tones are not sufficiently controlled as will be obvious from a diagram (FIG. 1) showing their directivity patterns. For example, shown in FIG. 1, within a range of 45.degree. toward the inside from the front position, dips of 10 to 15 dB exist at the frequencies of 3 kHz and 10 kHz and their angles also differ. Thus, the sound image moves in the listening area every frequency. It can be said that the technique does not match with the purpose of the design.
On the other hand, the sound output system according to the patent application of the present inventor mentioned above is the new sound output system with respect to the point that the directional distribution in a wide area can be certainly controlled by adjusting the shape and arrangement of the audio mirror. The technique such that the azimuth near a special direction is controlled in accordance with the object and use of the speaker. This invention also includes the technique of the variable type sound output system such that the directional distribution can be controlled in accordance with a environment of listening room. The present inventor has examined the extent of the listening area in which the stereophonic sound feeling is obtained in many embodiments. Thus, the following conclusions were obtained.
(a) As compared with the ordinary speaker system, the stereophonic listening area is obviously wide.
(b) However, with regard to the localization of the central sound source, in the case of what is called an omnidirectional speaker system, it is relatively limited to the position near the hot spot. On the other hand, when the directional speaker system is set so as to be localized to a special direction, the sweet area is obviously fairly widened.
As described above, in the case of the speaker system using the audio mirror, the listening area where the stereophonic sound feeling is derived is obviously wide. However, it is clear that the sweet area which can also localize the central sound source is influenced by the other conditions.